In the beginning, the earth's atmosphere had no oxygen. Anaerobic organisms, which can live without oxygen, were thriving. About 2.5 billion years ago, blue-green algae in the ocean acquired the ability to split water into hydrogen and oxygen and this chemical reaction initiated the release of oxygen into the atmosphere. The increased levels of atmospheric oxygen caused extinction of many anaerobic organisms owing to oxygen's toxicity. This important biological event also led to the evolution of multicellular organisms, including humans, who utilize oxygen for survival. The content of oxygen in the air gradually increased to the current amounts of about 21 percent in dry air and about 34 percent in water. The use of oxygen by any organism generates free radicals that are toxic. Therefore, during this period of atmospheric oxygenation, organisms must have struggled to survive the sudden exposure to oxygen toxicity. There must have been enormous rearranging of nucleotides in genes to produce specific proteins that could protect organisms against the damage produced by free radicals.
This process eventually led to the production of three antioxidant enzymes. Superoxide dismutase (SOD) requires manganese, copper, or zinc for its biological activity. Mn-SOD is present in mitochondria, whereas Cu-SOD and Zn-SOD are present in the cytoplasm and nucleus of the cell. All three can destroy free radicals and hydrogen peroxide. Another enzyme, catalase, requires iron for its biological activity and it destroys H2O2 in cells. Human tissue also contains glutathione peroxidase which requires selenium for its biological activity. It is also responsible for removing hydrogen peroxide.
Although iron, copper, and manganese are essential for the activities of antioxidant enzymes, a slight excess of free iron, Cu, or Mn can increase the production of free radicals, and subsequently enhance the risk of various chronic diseases. In addition, organisms, including mammals, consume certain antioxidants that are needed for growth and survival from plant sources. These antioxidants include carotenoids, vitamins A, C, and E, flavonoids, polyphenols, and herbal antioxidants.
Currently, the doses of antioxidants for the greatest benefit to human health are not well established. Nevertheless, increasing numbers of people are taking some form of supplements in the hope that it will optimize their health. Unfortunately, at present, they rely on advice from health-related magazines, books, advertising, radio and television reports or vitamin store salespeople. In fact, most people consume these nutrients without any scientific rationale. Furthermore, the majority of vitamin/mineral preparations have not given adequate attention to the dose, type, and chemical form of antioxidants, and appropriate minerals and other micronutrients.